This study examined relationships among physical activity, body composition, and stress- and immunity-related variables in fifth grade children (10-11 yrs) in Southern Ontario. The 29 boys and 32 girls, who participated in the study, performed a 20m shuttle run for prediction of aerobic fitness. Bioelectrical impedance was used to assess relative body fat (%BF). Standardized questionnaires were used to determine physical activity related variables and frequency of Upper Respiratory Tract Infection (URTI). Resting saliva samples were collected and tested for resting cortisol and resting secretory Immunoglobulin A (SIgA). Subjects wore a pedometer for 48h to estimate their average total distance traveled per day. Secretory IgA was significantly correlated with reported URTIs but not related to salivary cortisol, physical activity, fitness level or relative body fat. Children who spent more time in sport activities and had higher aerobic fitness reported fewer "sick" days. Children with body fat higher than 25% reported significantly (p<0.05) more sick days than the rest of the cohort. There were no gender differences in SIgA, URTI frequency and cortisol levels. The test-retest reproducibility for salivary cortisol was 0.66 (p<0.01), while long term SIgA reproducibility was non-significant for repeated measurements taken after six weeks. Resting secretory immunity was not strongly related to fitness and physical activity, but there was evidence that reduced physical activity and excess body fat can result in higher URTI incidence.